1. Field of the Invention
The present invention relates to a reflector, and in particular to that for reflection-type liquid-crystal display devices. It also relates to a liquid-crystal display device, and in particular to a reflection-type liquid-crystal display device.
2. Discussion of the Background
The recent tendency in the art is toward new display devices which are substituted for a conventional CRT, and a liquid-crystal display device (hereinafter referred to as an LCD) is one of them. LCDs are widely used in various fields, for example, as those for Office Automated (OA) apparatus, such as personal computers, word processors, work stations, etc.; those for electronic calculators, electronic books, electronic notebooks, personal digital assistant (PDA) apparatus, etc.; and those for portable TVs, portable telephones, portable facsimiles, etc.
In particular, display devices for portable apparatus are generally required to consume a small amount of power, because they are driven by batteries or the like. In addition, LCDs can be small-sized, thinned, and driven by a small amount of power, and are becoming much more popular.
Liquid crystals do not emit light by themselves. That is, liquid crystals are non-emitting display elements. Conventional LCDs are grouped into transmission-type ones and reflection-type ones.
The transmission-type LCD includes a flat lighting element, called a "back-light", at a back surface of the liquid-crystal panel. This type has been the mainstream in the art. However, the back-light consumes a relatively large amount of power, and therefore interferes with the advantage of low-power operability intrinsic to LCDs.
On the other hand, the reflection-type LCD includes a reflector for reflecting light at a back surface of the liquid-crystal panel, in which ambient light is reflected on the reflector to display images. This does not require a back-light, and therefore reduces the amount of required power. However, in conventional reflection-type LCDs, the liquid-crystal member has a low transmittance of a few percent to about ten percent. Therefore, the ambient light reflection cannot produce satisfactory brightness. For example, the devices cannot display bright paper white, and cannot display vivid color images. For these reasons, reflection-type LCDs have not been put into practical use in various fields, except for specific applications, such as wristwatches, electronic calculators, etc.
However, with the recent development of portable apparatus, there is a significant increase in demand for power-saving display devices. Therefore, the necessity of reflection-type LCDs has been taken into consideration. In particular, because reflection-type LCDs do not require a back-light and are small-sized, thin, and can be driven with a small amount of power, they are suitable for portable apparatus.
In display devices, a brightness of the picture screen is important. Specifically, in reflection-type LCDs, a reflectivity of the reflector is one important key point. As discussed above, the light transmittance of liquid crystals is not high. Therefore, to ensure satisfactory display quality, reflection-type LCDs must be provided with a high-quality reflector having a high reflectivity.
FIG. 19A and FIG. 19B are schematic views showing the reflection characteristic of reflectors.
Provided on each reflector is a liquid-crystal layer for on/off lighting. The liquid-crystal layer may be any type of TN mode, STN mode or the like having a polarizing plate, or of GH mode, PDLC mode or the like not having a polarizing plate. However, as its transmittance is generally low, the layer is problematic in that it cannot produce high brightness enough for display devices.
In addition, to display color, additive color mixing is generally employed, in which pixels of the three primary colors of light (RGB) are arranged in a plane configuration. In this system, the degree of light utilization is theoretically at most one third of that in monochromatic displaying. Therefore, reflectors with a perfect diffuse reflection characteristic cannot produce bright display images.
Ideally, it is desirable that the reflection characteristic of the reflectors used in reflection-type LCDs be a perfect diffuse reflection type as shown in FIG. 19A. Using a reflector with such a perfect diffuse reflection characteristic may give reflection-type LCDs which produce display images having a constant degree of brightness irrespective of a viewing direction. In this case, however, the incident light will scatter, and therefore the devices including a reflector of this type require strong incident light. Specifically, since the reflection intensity of the perfect diffuse reflection-type reflector is low, another problem contradictory to the problem with the devices as noted above occurs, in that the devices cannot have a satisfactory degree of brightness because of the low transmittance of the liquid crystals.
In most cases, an individual views a liquid-crystal panel almost at a direction perpendicular to the panel (i.e., at its front portion). Since the perfect diffuse reflection-type reflector diffuses light in a broad range, the intensity of the light from the reflector in the direction toward the front of the liquid-crystal panel is lower than a desired level.
To compensate the insufficiency of the light intensity for image displaying, a reflector having a light-scattering characteristic that is an intermediate between the perfect diffuse reflection characteristic and a specular reflection characteristic, as shown in FIG. 19B, may be employed. In general, the reflector with such a light-scattering characteristic can be realized by controlling a degree of roughness of a reflecting surface of the reflector. The reflector as controlled according to this method can increase the intensity of reflected light in some degree within a certain range around the center of the specular reflection direction. Within this range, the brightness of the reflected light from the light-scattering type reflector can be larger than that from the perfect diffuse reflection-type reflector.
However, devices provided with such a light-scattering type reflector are problematic in that the visible angle range is narrow. Therefore, the angle between the viewing direction and the panel must be delicately controlled relative to ambient lighting from ceiling lights and others. For these reasons, these devices are inconvenient for practical use. That is, lighting conditions around display devices vary all the time, for example, in indoor, outdoor, daytime or night time operation. Depending on the lighting conditions, therefore, there will occur still another problem with the devices in that a satisfactory degree of lighting can not be obtained for operation of display devices. The above-noted light-scattering type reflector can produce a higher degree of reflection intensity in specific directions than that to be produced by other perfect diffuse reflection-type reflectors. However, the former is still defective in that the lighting from it in the area overstepping the specific range is extremely dark and the visible angle range for it is narrowed.
The matter will now be described in more detail.
In FIG. 20, which shows the reflection characteristic of a conventional reflector, an incident ray (a) reflects on a reflecting surface 5 nearly in a front direction. An incident ray (b), as reflecting nearly on the top of a hill of the reflecting surface 5, reflects nearly in a specular direction. An incident ray having an optical path (c) reflects on an inclined area downstream from the top of the hill of the reflecting surface 5, and its reflection angle relative to the surface 5 is larger than its incident angle Oi. In other words, the incident ray (c) reflects on the surface 5 in a direction extremely shifted from the front direction, which is a viewing direction. As a result, the component (c) can not effectively contribute to a displayed image.
In addition, large reflection angles cause another problem as shown in FIG. 21. FIG. 21 shows optical paths of reflected rays in an LCD. Specifically, in FIG. 21, since the incident rays (a) and (b) reflect on the reflecting surface 5 at small reflection angles, almost all the reflected rays transmit out of the panel after passing through a counter substrate 8, a counter electrode 9, and an alignment film 10. However, the incident ray (c) that reflects on the surface 5 at a large reflection angle is problematic in that the reflected ray is totally reflected at an interface of the counter electrode 9 or the counter substrate 8, and therefore does not transmit out of the panel. Therefore, this component (c) does not contribute at all to displaying an image, and is nothing but light loss.
The critical angle for total reflection depends on the refractive indices of the two substances forming the interface. In ordinary liquid-crystal panels, the counter substrate 8 has a refractive index of about 1.55 or so. Therefore, it can be calculated that the angle for total reflection on the interface between the counter substrate 8 and air would be about 40.degree. or so. Accordingly, the component for which the reflection angle is 40.degree. or larger must be minimized as much as possible.
For these reasons, in order to improve the display quality of reflection-type liquid-crystal display devices, it is an important theme to obtain a reflector having a satisfactory degree of reflection intensity capable of compensating for the low transmittance of liquid crystals while ensuring a wide viewing angle range.
As mentioned above, controlling the reflecting orientation of the reflector to enhance the light-scattering ability thereof in conventional reflection-type LCDs often results in an insufficiency of the reflected light intensity in the front direction, which is the viewing direction, whereby the images produced therein are often dark. On the contrary, if the light-scattering ability of the reflector is attenuated, the intensity of reflected rays in specific directions can be increased, but there occurs a problem in that the viewing angle range is narrowed. Therefore, a wide viewing angle range and strong reflection are contradictory to each other in reflection-type LCDs. Accordingly, it has previously been difficult to display quality images.